[a_cool_username]

I can't give you any links because it's not open source code, but there was a bug fix in 0.790 named "Don't simplify away Any when joining union types" that caused me some problems with bad annotations in the existing code: The annotations implied that Any was possible, but it wasn't, but it got dropped from the final Union by the bug so we never had to handle Any. Dataclasses have had some backwards-incompatible improvements as well.

But the big culprit is typeshed. Something will get new/fixed annotations and suddenly you aren't handling all possible return types in your callers, or whatever.